Slice the beef thinly into strips about 2 inches long and put in a medium bowl. Combine the lemongrass, garlic, shallots, fish sauce, sugar, soy sauce, and sesame oil in a mini-food processor or a blender and blend until fairly smooth. (Add a little water if needed to move the blades.) Transfer the marinade to the bowl, toss the beef to coat evenly, and set aside for 30 minutes to 1 hour, or cover and refrigerate for up to 1 day.

To cook the beef, thread it onto bamboo skewers, pushing pieces together to fill most of the skewer, and leaving about 2 inches at each end. Build a hot charcoal fire or preheat a gas grill or the broiler. To grill, place the skewers on the hot grill and cook for 1 to 2 minutes on each side, using tongs or a long-handled spatula and grill fork to move the skewers off the grill as soon as they are done. To broil, place the skewers in a roasting pan and slide under the broiler until cooked, 1 to 2 minutes per side. You could also sauté the marinated beef quickly in a hot pan with 2 tablespoons of oil, turning once. Transfer the beef to a serving platter and serve hot or warm.